Computer display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a computer display screen or portion thereof with a transitional graphical user interface showing a first image in a first sequence according to the claimed design;

FIG. 2 is a front view showing a second image thereof;

FIG. 3 is a front view showing a third image thereof;

FIG. 4 is a front view of a computer display screen or portion thereof with a transitional graphical user interface showing a first image in a second sequence according to the claimed design;

FIG. 5 is a front view showing a second image thereof;

FIG. 6 is a front view showing a third image thereof; and, FIG. 7 is a front view showing a fourth image thereof.

The longer broken-lines showing of a portion of a computer and the intermediate-length broken-lines showing a perimeter of a display screen of the computer, as well as the features within the interface shown in broken-lines, are included for the purpose of illustrating environmental structure and form no part of the claimed design. 

CLAIM the ornamental design for a computer display screen or portion thereof with a transitional graphical user interface, as shown and described. 